in or to the greatest extent or degree (in this sense often used before adjectives and adverbs, and regularly before those of more than two syllables, to form superlative phrases having the same force and effect as the superlative degree formed by the termination -est):

11. The adverb most, a shortened form of almost, is far from being either a recent development or an Americanism. It goes back to the 16th century in England, where it is now principally a dialect form. In American English it occurs before such pronouns as all, anyone, anybody, everyone, and everybody; the adjectives all, any, and every; and adverbs like anywhere and everywhere: Most everyone around here is related to everyone else. You can find that plant most anywhere. This use of most is often objected to, but it is common in the informal speech of educated persons. It is less common in edited writing except in representations of speech.

most

adj.

Old English mast "greatest number, amount, extent," earlier mæst, from Proto-Germanic *maistaz (cf. Old Saxon mest, Old Frisian mast, Old Norse mestr, Dutch meest, German meist, Gothic maists "most"), superlative form of Proto-Germanic *maiz, root of Old English ma, mara (see more). Used in Old English as superlative of micel "great, large" (see mickle). Vowel influenced by more. Original sense of "greatest" survives in phrase for the most part (c.1400). Slang meaning "the best, extremely good" is attested from 1953. Also used as an adverb in Old English. Phrase make the most of (something) is by 1520s. Related: Mostly. Double superlative mostest is 1885, from U.S. Southern and Black English.

at most

Also,at the most or at the outside. At the largest amount, the furthest limit; also, in the most extreme case. For example, She'll be finished in two weeks at the most, or It'll take two weeks at the outside, or At most the chef uses a tiny bit of pepper. The terms with most date from the 1300s; at the outside from the mid-1800s. Also see at best